My outright guess is no; a counterargument would be that if everyone did this, then because there aren't enough such seats, then it wouldn't be fair to the peope who failed to perceive these empty seats.

Yet what would be some arguments in favour? Has this happened? Those empty seats feel wasteful.

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The airline also has to provide you with food, service and other resources thereby dividing the crew's attention from people who actually paid for the ticket.
– Aditya SomaniJul 30 '14 at 14:34

5

I believe the term commonly used is "Self Upgraders", and they are normally spotted and ejected back to their normal seats...
– GagravarrJul 30 '14 at 14:39

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Your chances are higher if your boarding pass shows you're a frequent flyer on their airline. Often there's an asterisk or other subtle indication that you're not total riff-raff.
– Spehro PefhanyJul 30 '14 at 15:34

3 Answers
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You are not permitted to sit in a different cabin from your ticketed cabin unless you are moved there by the cabin crew (say, there is a medical problem and extra seats are needed mid-flight— though you'd still probably get reseated in crew seats instead of in business class). "Self-upgraders" will be discovered very quickly and removed as a matter of policy on any airline.

Why not allow people to move to empty seats? One might as well ask why not allow people at the gate to fill empty seats in the economy cabin. The plane is leaving one way or the other, so those seats would be "wasted" too. But the airline must protect its revenue and minimize its costs. If it tolerated people moving up without paying, it would both disincent people from purchasing premium fares (whether revenue or award), and also incur costs that it would not otherwise have had to bear.

Seats on an airplane are priced according to how the airline believes it can make a profit, taking many factors into consideration. Premium cabin prices reflect the premium people (or rather more often, their employers) are willing to pay for a more comfortable flight and a more exclusive cabin, as well as the higher costs of operating:

There are fewer seats for the given space, so each seat must cost more to make the equivalent revenue

There is a higher grade of service in those cabins; for example, there is a higher ratio of flight attendants to passengers, and the flight attendants have more responsibilities (e.g. addressing passengers by name, distributing pajamas). Flight attendants are expensive.

The amenities in premium cabins are more expensive, not only to purchase but to prepare. In economy, your meal probably comes on trays that come straight out of the oven. In international first, you would expect your meal to be plated, which takes much more time and effort.

It is already the case that when the economy cabin is full, gate agents will upgrade people who are high status frequent flyers and/or on very high fare tickets, according to each airline's rules and procedures (operational upgrades). American, Delta, and United indeed offer "automatic" upgrades to their elite frequent flyers on domestic flights. But even in these cases, some seats may go out empty. For example, the airline may be unable to cater enough meals for a full premium cabin, and the airline would not want to risk upsetting a paying customer by failing to deliver the promised level of service.

I think the initial statements about the revenue are correct, but only as long as that higher classes often/usually have empty seats. In my country, moving to a seat of a higher class in any means of public transportation (e.g. planes or trains) is forbidden (and those are frequently not fully booked out, so there would actually be a good chance of getting a seat there). On the other hand, using a train seat without paying for a seat reservation, or paying the minimum price in a theatre and then moving to an unoccupied, but more expensive seat, is accepted, as that works only at certain times.
– O. R. MapperJul 30 '14 at 18:19

@O.R.Mapper I am speaking principally of international flights. There are of course situations where it is easy to do as you say. It is very common in the U.S., for instance, for people to move to better seats at sporting events midway through a game/match— though the most exclusive areas remain controlled for the duration.
– chosterJul 30 '14 at 18:26

But coming back to the question: If that is actually validly possible, that passengers can get assigned several seats at a time even though paying for just one, it would seem very contradictory if ticket holders weren't allowed to switch the one seat they paid for for another one in the same price category.
– O. R. MapperJul 30 '14 at 19:01

"Those empty seats feel wasteful." Ultimately, moving someone from economy to premium economy or to business once the plane is loaded doesn't change the number of empty seats, it only changes the location of the empty seats. The plane is still just as full as before and the airline's revenue is the same, so there is no real motivation for the airline to move you.

There are cases where someone maybe assigned a premium economy seat without paying for it, when the plane is full and the only remaining empty seats when they check in are in the premium section. But that is done by ground staff out of necessity, not out of kindness once the airplane is boarded.

"no real motivation for the airline to move you" - that depends on whether making customers as happy as possible counts as a motivation. (I do see some possible specific counterarguments for airlines, such as choosing who spontaneously gets the better seat, and also the possibility that a part of the increased quality of higher class seats is exactly that it's not so crowded there due to some free seats in between.)
– O. R. MapperJul 30 '14 at 18:22

1

As choster mentioned there are upgrade programs for elite flyers offered on many airlines, but these tend to be controlled by the ground staff / corporate rules before boarding. These are customer service oriented decisions designed to encourage return customers. But for non-frequent flyers there is no way to determine who is most worthy of courting, who is most worthy of upgrading, so it is easier for the airlines to not upgrade any of them.
– user13044Jul 30 '14 at 18:49

I and two family members traveling together were once upgraded from "last class" to first class (in a 747) flying from Seattle to London. This was 1989, so YMMV.

We arrived at the Seattle departure desk about 40 minutes before the cut-off time 45(?) minutes before departure, but had to join a long queue. At right about the cut-off time, we got to the desk. The agent told me the flight is full. I glanced at the clock and said "Well, we're here on time; what are you going to do about it?" She seemed none-too-pleased, but clacked away at the keyboard for a minute or two. I was extremely curious what she was doing, but uncharacteristically remained quiet. She then provided three new tickets (which was how boarding passes were done then), and said "I'm upgrading you to first class. Have a nice flight."

I had negotiated aggressively months before for excellent fares; likely we paid among the least of anyone on that flight, approximately $375 (round trip) per person for a late August two week trip when the going rate was $675–750. My mom was seated next to a highly opinionated London businessman. When they got to discussing ticket prices he asked for proof and then harangued a flight attendant mercilessly for charging him $3,500 for a one way ticket and being seated next to someone paying less than a tenth of what he did.

I expected that for the low fare we paid, we'd still get coach food service, but no: we got all the goodies (travel kit, complementary towel, etc.), food, and drink—like free cocktails and wine—just like all the other first class passengers.

No such luck on the return flight, and it hasn't happened to me since even after traveling for more than four million miles.